Surface emission type semiconductor laser

ABSTRACT

A surface emission type semiconductor laser includes a plurality of semiconductor layers defining at least one resonator in a direction perpendicular to the semiconductor substrate of the laser, the layers including at least a cladding layer in the semiconductor layers being formed into at least one column-like light emitting portion extending in a direction perpendicular to the semiconductor substrate, and a II-VI group compound semiconductor epitaxial layer buried around the column-like portion. An active layer of multi-quantum well structure is further formed on the layer section of the cladding layer having the column-like portion. If a plurality of column-like portions are formed, these column-like portions are separated from one another by a separation groove terminating short of the active layer, the II-VI group compound semiconductor epitaxial layer being buried in the separation groove. Particularly, when it is desired to produce a phase synchronization type semiconductor laser, a waveguide layer is formed below the active layer, and the waveguide layer is adapted to propagate light rays in a direction parallel to the active layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface emission type semiconductorlaser adapted to emit a laser beam in a direction perpendicular to thesubstrate thereof.

2. Description of the Related Art

A surface emission type laser including a resonator disposed in adirection perpendicular to the substrate thereof is disclosed inLectures of the 50-th Meeting of Applied Physics in Japan (1989), Vol.3, p. 909, 29a-ZG-7. In accordance with the prior art, as shown in FIG.12, there is first provided an n-type GaAs substrate 602 on which ann-type AlGaAs/AlAs multi-layer film 603, an n-type AlGaAs cladding layer604, a p-type GaAs active layer 605 and a p-type AlGaAs cladding layer606 are sequentially grown and formed. The multi-layered structured isthen etched while leaving a column-like part at the top thereof. Theremaining column-like part is enclosed by a buried layer which is formedby sequentially growing a p-type layer 607, n-type layer 608, p-typelayer 609 and p-type layer 610 all of which are of AlGaAs in liquidphase epitaxy method. Thereafter, a multi-layer dielectric film 611 isdeposited on the cap layer of p-type AlGaAs 610 at the top thereof.Finally, p- and n-type ohmic electrodes 612 and 601 are formedrespectively on the top and bottom end faces of the structure thusformed. In such a manner, a surface emission type semiconductor laserwill be completed.

The buried layer (607-608) formed in the above manner defines a p-njunction which is used as means for preventing current from leaking tolayer sections other than the active layer section.

However, by using such a p-n junction, it is difficult to provide asufficient current restriction; and it cannot suppress any reactivecurrent perfectly. Due to generation of heat in the component,therefore, the surface emission type semiconductor laser constructed inaccordance with the prior art is impractical in that it is difficult toperform a continuous generating drive in room temperature. It is thusimportant to restrict the reactive current in the surface emission typesemiconductor laser.

Where the buried layer is of a multi-layered structure to form a p-njunction as in the prior art, the p-n interface in the buried layershould be positioned in consideration of a position of the interfacebetween each of the adjacent column-like grown layers. It is difficultto control the thickness of each layer in the multi-layered structure.It is therefore very difficult to consistently produce surface emissiontype semiconductor lasers.

If a buried layer is formed around the column by the liquid-phaseepitaxy method as in the prior art, there is a high risk of breaking-offof the column-like part, leading to a reduced yield. The prior art wasthus subject to a structural limitation in improving itscharacteristics.

The prior art raises further problems even when it is applied to variousother devices such as laser printers and the like.

For example, laser printers can have an increased freedom of design asin simplifying the optical system or in decreasing the optical path,since the source of light (semiconductor laser and so on) has arelatively large size of light spot equal to several tens μm and if alight emitting element having an increased intensity of light emissionis used in the laser printers.

With the surface emission type semiconductor laser constructed accordingto the prior art, the optical resonator is entirely buried in a materialhaving a refractive index higher than that of the resonator. Light raysare mainly guided in the vertical direction. As a result, a spot oflight emission in the basic generation mode will have a diameter equalto about 2 μm even if the shape of the resonator is modified in thehorizontal direction.

It has been proposed that the light spots be located close to each otherup to about 2 μm and that a plurality of light sources be used toincrease the size of a spot. From the standpoint of reproductiveness andyield, however, it is very difficult with the prior art to bury aplurality of resonators spaced away from one another by several micronsusing the LPE method. Even if such a burying can be successfully carriedout, the spots cannot be united into a single spot since the transverseleakage of light is little.

It is also necessary that a plurality of light spots are formed into asingle beam of light and that the laser beams each consisted of pluralspots are in phase to increase the intensity of light emission. However,the prior art could not produce a surface emission type semiconductorlaser which emits a plurality of laser beams close to one another up toa distance by which one of the laser beams are influenced by the other,in order to synchronize the laser beams in phase.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide ahigh-efficiency surface emission type semiconductor laser which is of acomplete current restrictable structure provided by appropriatelyselecting the material of the buried layer and by improving thestructure of the active layer and also which can be produced in a verysimple manner.

Another object of the present invention is to provide a surface emissiontype semiconductor laser which includes a plurality of light emittingportions close to one another and which can synchronize laser beams fromthe light emitting portions in phase and which can be continuouslydriven in room temperature.

Still another object of the present invention is to provide a surfaceemission type semiconductor laser which can emit a single laser beamformed by the in-phase laser beams from the light emitting portions,said single laser beam having a relatively large light spot and areduced angle of radiation.

To this end, a surface emission type semiconductor laser for emitting abeam in a direction perpendicular to the semiconductor substrate,comprises an optical resonator including a pair of reflecting mirrorshaving different reflectivities and a plurality of semiconductor layerslocated between the reflecting mirrors, the layers including at least acladding layer in said semiconductor layers being formed into one ormore column-like semiconductor layers (portions); a II-VI group compoundsemiconductor epitaxial layer buried around (surrounding) saidcolumn-like semiconductor layers; and an active layer of multi-quantumwell structure formed at a lower layer section of said cladding layerhaving the one or more column-like portions, said active layer includinga plurality of well layers each of which is sandwiched between barrierlayers.

The II-VI group compound semiconductor epitaxial layer may be formed ofa combination of two, three or four elements which include II-groupelements such as Zn, Cd and Hg and VI-group elements such as O, S, Seand Te. It is also desirable that the lattice constant of the II-VIgroup compound semiconductor epitaxial layer is equal to that of thecolumn-like semiconductor layers. It is preferred that the semiconductorlayer defining the resonator is a III-V group compound semiconductorepitaxial layer of GaAs, GaAlAs, GaAsP, InGaP, InGaAsP, InGaAs, AlGaAsSbor the like.

Since the II-VI group compound semiconductor epitaxial layer has a highresistance, the buried layer formed by this high-resistance layer canprevent a leakage of incoming current thereinto. This can attain veryeffective current restriction. Furthermore, the threshold level ofcurrent can be decreased since the reactive current is reduced. Inaddition, the active layer of multi-quantum well (MQW) structure alsoserves to reduce the generating threshold current. By the appropriateselection of the buried material and the modification of the activelayer, the present invention can provide a surface emission typesemiconductor laser which generates less heat and can continuouslyperform the generation in room temperature. Since the buried layer isnot multi-layered, it can be easily formed with consistency. If theactive layer is of MQW structure, its gain is increased with the lightoutput being increased. If the material of the active layer is changedfrom one to another, the wavelength of generation also is naturallyvaried. However, the present invention performs the change of thewavelength by varying the MQW structure while using the same material.Furthermore, the II-VI group compound semiconductor epitaxial layer canbe formed by any other suitable manner other than the liquid-phaseepitaxy method, such as vapor-phase epitaxy method, resulting inimprovement of the yield in forming the column-like semiconductorlayers. If the vapor-phase epitaxy method is used, a buried layer can bereliably formed while permitting a plurality of column-likesemiconductor layers to be arranged closer to one another, even if aspace in which the buried layer is to be formed is small.

If the thickness of the semiconductor contact layer on the exit side ofthe optical resonator is equal to or less than 3.0 μm, the lightabsorption can be reduced in the contact layer.

If the cross-section of the column-like semiconductor layer parallel tothe semiconductor substrate is circular or regular polygonal, it canprovide a fairly circular spot beam. If the diameter or diagonal of thecross-section just mentioned is equal to or less than 10 μm, NFP (NearField Pattern) mode becomes 0-order basic mode.

Since the active layer of MQW structure is very thin, it is preferredthat a waveguide layer is provided to propagate the light rays in adirection parallel to the active layer.

If the optical resonator has a single column-like portion, thereflecting mirror on the exit side thereof may be formed at a positionopposite to the end face of the column within the range of said endface. In this case, a refractive index waveguide structure of ribwaveguide type may be provided by the active layer of MQW structure. Ifthe column-like portion includes an active layer of MQW structure, arefractive index waveguide structure of buried type may be realized.

In this surface emission type semiconductor laser, the optical resonatormay include separation groove(s) for separating one of the column-likeportions from another adjacent one. The II-VI group compoundsemiconductor epitaxial layer is buried in the separation groove and alight emitting portion is formed on each of the column-like portions.When the active layer of MQW structure is located so as not to reach theseparation groove, the light emitting portions will be influenced byeach other such that the light rays from the respective light emittingportions are synchronized with each other in phase. Even though aplurality of column-like portions arranged in a two-dimensional arrayare formed within a finely small area, the present invention can reducethe reactive current such that the surface emission type semiconductorlaser can be continuously driven in room temperature. In order toincrease the advantage of the phase synchronization, a waveguide layeris preferably located at the lower layer section of the active layer ofMQW structure. In this case, the influence between the light emittingportions will be emphasized to facilitate the phase synchronization, forexample, even if the respective light emitting portions are spacedfarther away from one another.

When it is desired to increase the light emission spot, a II-VI groupcompound semiconductor epitaxial layer which is transparent for thewavelength of the exit laser beam may be buried in the separationgroove. The exit side reflecting mirror is formed through a regionopposite to the end face of each of the column-like portions and theII-VI group compound semiconductor epitaxial layer buried in theseparation groove. Thus, a region sandwiched between each adjacent lightemitting portion also serves as a vertical resonating structure. Lightleaked into such a region effectively contributes to the lasergeneration to increase the light emission spot in size. Since thesynchronized laser beams are superimposed one over another, the lightoutput increases and the angle of radiation decreases. With a GaAs lasergenerally used as a semiconductor layer of a resonator, the II-VI groupcompound semiconductor epitaxial layer transparent for the wavelength ofthe laser beam therefrom may be made of either ZnSe, ZnS, ZnSSe, ZnCdSor CdSSe. If the separation groove is perpendicular to the semiconductorsubstrate, light rays slantingly entering the separation groove can betotally reflected to increase the confinement of light, utilizing adifferential refraction. If the cross-section of the separation grooveparallel to the semiconductor substrate has a width ranging between 0.5μm and 10 μm, the order of the transverse generation mode measured fromNFP becomes 0-order basic mode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view, partially in section, of a light emittingportion of one embodiment of a semiconductor laser constructed inaccordance with the present invention.

FIG. 2 is an enlarged cross-sectional view of the active MQW layer inthe semiconductor laser shown in FIG. 1.

FIGS. 3a-3e are cross-sectional views illustrating the process of makingthe semiconductor laser shown in FIG. 1.

FIG. 4 is a graph illustrating the relationship between the drivecurrent and the generated light output in the semiconductor laser shownin FIG. 1.

FIG. 5 is a perspective view, partially in section, of a light emittingportion of another embodiment of a semiconductor laser constructed inaccordance with the present invention.

FIG. 6 is a schematic cross-section of a light emitting portion of asurface emission type semiconductor laser constructed in accordance withthe present invention, the semiconductor laser adapted to generate laserbeams synchronized in phase.

FIGS. 7a-7f cross-sectional views of the semiconductor laser shown inFIG. 6, illustrating the process of making it.

FIGS. 8a-8d illustrate differences in shape and near field patternbetween the surface emission type semiconductor laser constructedaccording to the prior art and the semiconductor laser of FIG. 6: FIG.8(a) shows the shape of the surface emission type semiconductor laser ofthe prior art on the exit side thereof; FIG. 8(b) shows an intensityprofile in the near field pattern of the semiconductor laser shown inFIG. 8(a); FIG. 8(c) shows the shape of the semiconductor laser of thepresent embodiment at the exit side; and FIG. 8(d) shows an intensityprofile of the near field pattern of the semiconductor laser shown inFIG. 8(c).

FIGS. 9(a) to (m) schematically illustrate various shapes of surfaceemission type semiconductor lasers constructed according to furtherembodiments of the present invention at the exit sides thereof.

FIGS. 10(a) to (d) schematically illustrate various shapes of surfaceemission type semiconductor lasers constructed according to stillfurther embodiments of the present invention at the exit sides thereof.

FIGS. 11(a) to (c) schematically illustrate various shapes of surfaceemission type semiconductor lasers constructed according to otherembodiments of the present invention at the exit sides thereof.

FIG. 12 is a perspective view of a surface emission type semiconductorlaser constructed according to the prior art, illustrating the lightemitting portion thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 2, there is shown a semiconductor laser 100constructed in accordance with one embodiment of the present invention.

The semiconductor laser 100 comprises a substrate of n-type GaAs 102over which a buffer layer of n-type GaAs 103 is formed. Over the bufferlayer 103, there are formed 30 pairs of distribution reflection typemulti-layer film mirror 104 which include an n-type Al₀.7 Ga₀.3 As layerand an n-type Al₀.1 Ga₀.9 As layer and have a reflectivity equal to orhigher than 98% against light rays having their wavelength substantiallyequal to 870 nm. On the multi-layer film mirror 104, there aresequentially formed a cladding layer of n-type Al₀.4 Ga₀.6 As 105, anactive layer of multi-quantum well structure 106, another cladding layerof p-type Al₀.4 Ga₀.6 As 107 and a contact layer of p-type Al₀.1 Ga₀.9As 108, utilizing the epitaxial growth in the MOCVD process (see FIG.3(a)). For example, the active MQW layer 106 comprises three well layers106a, as shown in FIG. 2. Each of the well layers 106a is sandwichedbetween a pair of upper and lower barrier layers 106b. Each of the welllayers 106a may be formed as of an i-type GaAs film having a thicknessequal to 120 Angstroms while each of the barrier layers 106b may beformed as of an i-type Ga₀.65 Al₀.35 As film having a thickness equal to150 Angstroms. The MOCVD process was carried out under such a conditionthat the temperature on growth is 700° C. and the pressure on growth is150 Torr. and used organic metals such as TMGa (trimethyl gallium) andTMAl (trimethyl aluminum) as III-group materials, AsH₃ as V-groupmaterial, H₂ Se as n-type dopant and DEZn (diethyl zinc) as n-typedopant.

After growth, an SiO₂ layer 112 is formed on the top of themulti-layered structure by the thermal CVD process. By the use of thereactive ion beam etching process (RIBE process), the multi-layeredstructure is etched up to the middle of the cladding layer of p-typeAl₀.4 Ga₀.6 As 107, leaving a column-like light emitting portion coveredwith a hard baked resist layer 113 (FIG. 3(b)). The etching gas is amixture of chlorine with argon under a pressure of 1×10⁻³ Torr. Theleading voltage used therein is equal to 400 V. The purpose of theetching of the cladding layer 107 up to the middle thereof is to providea rib waveguide type refraction waveguide structure which can confineinjected carriers and light rays in the active layer.

After the resist 113 has been removed, a layer of ZnS₀.06 Se₀.94 109which is in lattice alignment with GaAs is then formed by causing thislayer to grow around the column-like portion using the MBE or MOCVDprocess (FIG. 3(c)).

Four pairs of multi-layered dielectric film mirror of SiO₂ /α-Si 111 arethen formed on the top of the column-like portion by the use of electronbeam deposition. Dry-etching is then used to remove a part of the mirror111, leaving a region slightly smaller than the diameter of the lightemitting portion (FIG. 3(d)). The reflectivity of the multi-layereddielectric film mirror 111 is 94% for wavelength of 870 nm.

Thereafter, a p-type ohmic electrode 110 is deposited on all the topface except the multi-layered dielectric film mirror 111. An n-typeohmic electrode 101 is further deposited over the bottom face of thesemiconductor substrate 102. In an atmosphere of N₂, the entirestructure is alloyed at 420° C. to provide a surface emission typesemiconductor laser (FIG. 3(e)).

The surface emission type semiconductor laser thus formed provides avery effective current restriction since the buried layer 109 of ZnS₀.06Se₀.94 has a resistance equal to or higher than one G Ω and there is noinjection current into the buried layer. In this embodiment, further,the active MQW layer 106 contributes to the reduction of generatingthreshold current. Since it is not required that the buried layer is anymulti-layered structure, it can be more easily grown with an increasedreproductiveness (consistency) from one batch to another. Theutilization of the rib waveguide structure using the ZnS₀.06 Se₀.94layer having its refractive index sufficiently smaller than that of GaAscan realize a more effective light confinement.

FIG. 4 shows the relationship between the drive current and thegenerated light output in the surface emission type semiconductor laseraccording to this embodiment. In this embodiment, the continuousgeneration of laser beam can be accomplished in room temperature with athreshold level as low as 10 μA by using the buried layer of II-VI groupcompound semiconductor 109 and the active MQW layer 106. If any activeMQW layer is not used, the continuous generation of laser beam can beattained, but the threshold current will be as large as one mA. Theactive MQW layer serves to provide a light output at least five timeshigher than a case where no active layer of MQW structure is taken, forexample, 25 mW or higher. Furthermore, the external differential quantumefficiency is increased and the characteristics of the laser is improvedby restricting the reactive current.

If the cross-sectional shape of the column-like portion in the surfaceemission type semiconductor laser according to this embodiment of thepresent invention is of a circle or regular polygon such as square orregular octagon, a finely circular spot of beam can be provided.However, if this cross-sectional shape is of any configuration otherthan the above ones, such as rectangle or trapezoid, the cross-sectionof the laser beam will be ellipse or multi-mode. This is not desirablein applying the semiconductor laser to the devices.

                  TABLE 1    ______________________________________    Diameter of Column-Section                      Mode of Near Field Pattern    ______________________________________     2 μm          Zero-Order Basic Mode     5 μm          Zero-Order Basic Mode    10 μm          Zero-Order Basic Mode    12 μm          First-Order Mode    15 μm          First-Order Mode    ______________________________________

Table 1 shows the relationship of near field pattern relative to thediameter of the cross-section of the column-like portion in the surfaceemission type semiconductor laser according to this embodiment of thepresent invention. It will be apparent therefrom that if the diameter isequal to or less than 10 μm, the generation is carried out in the basicmode.

It is preferred that the contact layer 108 in the surface emission typesemiconductor laser according to this embodiment of the presentinvention is of a thickness equal to or less than 3.0 μm. This isbecause of reduction of the light absorption in the contact layer. Sucha thickness is more preferably less than 0.3 μm because the elementresistance is reduced and the external differential quantum efficiencyis increased.

The present invention can be applied to a buried type refractive indexwaveguide structure as shown in FIG. 5, rather than the aforementionedrib waveguide type refractive index waveguide structure. In this case,the column-like semiconductor layers are formed to extend to theunderlying cladding layer 105. As a result, the active MQW layer 106will also be formed into a column-like configuration and enclosed by theburied layer 109.

Referring next to FIGS. 6 and 7a-7f, there is shown still anotherembodiment of a phase-synchronization type semiconductor laser 300constructed in accordance with the present invention, which can increasethe dimension of the emission spot.

The semiconductor laser 300 comprises a substrate of n-type GaAs 302over which a buffer layer of n-type GaAs 303 is formed. Over the bufferlayer 303, there are formed 25 pairs of distribution reflection typemulti-layer film mirror 304 which includes an n-type Al₀.9 Ga₀.1 Aslayer and an n-type Al₀.2 Ga₀.8 As layer and have a reflectivity equalto or higher than 98% against light rays having their wavelengthsubstantially equal to 780±30 nm. On the multi-layer film mirror 304,there are sequentially formed a cladding layer of n-type Al₀.5 Ga₀.5 As305, a waveguide layer 315, an active layer of multi-quantum wellstructure 306, another cladding layer of p-type Al₀.5 Ga₀.5 As 307 and acontact layer of p-type Al₀.15 Ga₀.85 As 308, utilizing the epitaxialgrowth in the MOCVD process (see FIG. 7(a)). At this time, for example,the active MQW layer 306 comprises three well layers 306a, each of whichis sandwiched between a pair of upper and lower barrier layers 306b, asshown in FIG. 2. Each of the well layers 306a may be formed of an i-typeGa₀.65 Al₀.35 As film having a thickness equal to 80 Angstroms whileeach of the barrier layers 306b is made of an i-type Ga₀.95 Al₀.05 Asfilm having a thickness equal to 60 Angstroms. The waveguide layer 315has a composition of Al ranging between those of the well and barrierlayers 306a, 306b and may be formed of an n-type epitaxial layer ofGa₀.75 Al₀.25 As. Such a waveguide layer has a refractive index lowerthan the equivalent refractive index of the active MQW layer 306 buthigher than the refractive index of the underlying cladding layer 305.The MOCVD process may be carried out under such a condition that thetemperature on growth is 720° C. and the pressure on growth is 150 Torr.and use organic metals such as TMGa (trimethyl gallium) and TMAl(trimethyl aluminum) as III-group materials, AsH₃ as V-group material,H₂ Se as n-type dopant and DEZn (diethyl zinc) as p-type dopant.

After the growth, an SiO₂ layer is formed on the top of themulti-layered structure in the atmospheric pressure by the thermal CVDprocess. A photoresist is then applied over the SiO₂ layer and baked ata raised temperature to form a hard baked resist. A further SiO₂ layeris formed over the hard baked resist by the EB deposition.

The respective layers formed on the substrate are then etched by the useof reactive ion etching process (RIE process). The SiO₂ layer formed onthe hard baked resist 313 is first subjected to the conventionalphotolithograph to form a necessary resist pattern. This resist patternis then used as a mask to perform the RIE process against the SiO₂layer. For example, the RIE process may be carried out by using CF₄ gasunder a pressure of 4.5 Pa and an input RF power of 150 W and bycontrolling the sample holder at 20° C. This SiO₂ layer is then utilizedas a mask to etch the hard baked resist 313 by the RIE process whichuses, for example, O₂ gas under a pressure of 4.5 Pa and an input powerof 150 W and controls the sample holder at 20° C. At the same time, theresist pattern initially formed on the SiO₂ layer is also etched. Inorder to etch both the SiO₂ layer left in the pattern and the SiO₂ layer312 formed on the epitaxial layer simultaneously, the etching is againperformed by the use of CF₄ gas. By using the thin SiO₂ layer as a maskand performing the RIE process which is one of the dry etching processesagainst the hard baked resist 313, the latter may include side wallsperpendicular to the substrate while maintaining the necessary pattern(FIG. 7(b)).

The hard baked resist 313 having such vertical side walls is used as amask in the reactive ion beam etching (RIBE) process so that thecladding layer of p-type Al₀.5 Ga₀.5 As 307 is etched up to its middle,leaving a plurality of column-like light emitting portions (FIG. 7(c)).The etching gas used herein is a mixture of chlorine with argon under apressure equal to 5×10⁻⁴ Torr. and a plasma generating voltage equal to400 V. The RIBE process is carried out at the current density of ionequal to 400 μA/cm² on the etching sample while maintaining the sampleholder at 20° C. The purpose of etching the cladding layer 307 up to itsmiddle is to provide a refraction waveguide type rib waveguide structurefor confining the horizontal injection carriers and light rays in theactive MQW layer 306 such that a part of the light rays can betransmitted in the horizontal direction within the active layer. Also inthis embodiment, the propagation of light in the horizontal directionmay be assured by the waveguide layer 315.

If the RIBE process in which an ion beam is irradiated perpendicular tothe hard baked resist 313 having its vertical side walls and the etchingsample to etch them is used, the light emitting portions 320 arrangedclose to each other can be separated from each other by a separationgroove 314 and at the same time it is possible to produce a verticallight resonator which is required to improve the characteristics of thesurface emission type semiconductor laser.

After the hard baked resist 313 has been removed, the MBE or MOCVDprocess is used to grow a layer of ZnS₀.06 Se₀.94 309 around the lightemitting portions, such a buried layer serving as a II-VI group compoundepitaxial layer which is in lattice alignment with Al₀.5 Ga₀.5 As (FIG.7(d)). This buried layer 309 is transparent for the generationwavelength of the surface emission type semiconductor laser 300.

Next, the SiO₂ layer and polycrystalline ZnSSe produced thereon areremoved. Thereafter, four pairs of multi-layered dielectric film mirror311 made of SiO₂ /α-Si are formed on the top of the multi-layeredstructure by means of electron beam deposition. Dry etching is then usedto remove a part of the mirror 311 (FIG. 7(e)). The reflectivity of themulti-layered dielectric film mirror at wavelength of 780 nm is 95% ormore. Since the multi-layered dielectric film mirror 311 is also formedover the separation groove 314 buried with ZnSSe, a vertical resonatorstructure also is formed at the region between the adjacent lightemitting portions. As a result, light rays leaked into the separationgroove 314 effectively contributes to the laser generation. Since theleaked light rays are utilized, the emitted light can be synchronizedwith the phase at the light emitting portions 320.

Thereafter, a p-type ohmic electrode 310 is deposited on the top faceexcept the multi-layered dielectric film mirror 311. An n-type ohmicelectrode 301 is deposited on the bottom face of the substrate. Thestructure thus formed is alloyed at 420° C. in the atmosphere of N₂ tocomplete the surface emission type semiconductor laser 300 (FIG. 7(f)).The n-type ohmic electrode 310 on the exit side is formed to connectwith the contact layer 308 in each of the light emitting portions.

Since the surface emission type semiconductor laser produced accordingto this embodiment utilizes the epitaxial ZnSSe layer 309 as a buriedlayer, it can have a resistance equal to or higher than one GΩ, which ishigher than that of the prior art blocking structure using a counterbias at the p-n junction in the AlGaAs layer. This provides an optimumcurrent blocking structure. As a result, the generating thresholdcurrent can be reduced. In addition, the active MQW layer 306 alsoserves to reduce the generating threshold current, for example, to alevel of about ten μA. Furthermore, the light emitting portions 320separated from each other by the separation groove 314 can be influencedby each other through the active and waveguide layers 306 and 315. Thus,light rays from the respective light emitting portions 320 can besynchronized with each other in phase, resulting in generation of alaser beam having an increased diameter and an emphasized intensity.Moreover, the light leaked from the light emitting portion 320 can beeffectively utilized since the buried layer is made of a transparentmaterial having less absorption for the generation wavelength of 780 nm.

FIGS. 8a-8d show the arrangements of the surface emission typesemiconductor lasers constructed respectively in accordance with theprior art and the present invention at the exit sides thereof andintensity profiles of NFP when the laser beam is generated. FIG. 8(a)shows that the resonators 620 of the prior art surface emission typesemiconductor laser 600 shown in FIG. 12 are arranged close to oneanother up to a distance by which the resonators can be fully covered bythe epitaxial layers of GaAlAs 607 and 608 connected with each other atthe n-p junction, that is, a distance equal to about 5 μm. Although theexit face of the laser actually includes the multi-layered dielectricfilm mirror and the p-type ohmic electrode formed thereon, they areomitted in FIG. 8(a) for clear illustration. FIG. 8(b) shows anintensity profile of NFP between points a and b in FIG. 8(a). The priorart surface emission type semiconductor laser only provides a pluralityof adjacent light spots even if a plurality of light emitting portions620 are arranged closed to each other.

FIG. 8(c) shows the arrangement of the exit end of the surface emissiontype semiconductor laser constructed according to this embodimentwherein the separation groove is buried with a ZnS₀.06 Se₀.94 layer 309which formed by the vapor-phase epitaxy method. Thus, the minimum widthof the separation groove can be equal to one μm. FIG. 8(d) shows NFPbetween points c and d in FIG. 8(c). It will be apparent from this NFPthat the light emission spot is enlarged since light rays exit also fromabove the separation groove 314. Since the adjacent laser beams aresynchronized with each other in phase, the light output can be increasedwith an angle of radiation being equal to or less than one degree.

Table 2 shows the relationship between the width of the separationgroove of the surface emission type semiconductor laser 300 and theorder of transverse generation mode measured from NFP.

                  TABLE 2    ______________________________________    Width of Separation Groove                      Mode of Near Field Pattern    ______________________________________    0.5 μm         Zero-Order Basic Mode    1.0 μm         Zero-Order Basic Mode    5.0 μm         Zero-Order Basic Mode     10 μm         First-Order Mode     20 μm         Higher-Order Mode    ______________________________________

If the width of the separation groove is less than 10 μm, the transversegeneration mode of the laser synchronized in phase is in the basic mode.If the width is equal to or more than 10 μm, the laser will be generatedin an order equal to or higher than one. In this case, the laser beamwill be of an elliptic configuration with its increased angle ofradiation. This is undesirable in all the applications. If theseparation groove has a width less than 0.5 μm, the laser beam will notbe circular.

Although the embodiments have been described as to a single opticalresonator including a plurality of light emitting portions spaced awayfrom one another, a plurality of such optical resonators may be formedon the same semiconductor substrate. If each of the optical resonatorshas a p-type ohmic electrode at its exit side, a laser beam from eachoptical resonator may be independently controlled with respect to ON,OFF and modulation.

Although the embodiments have been described as to the surface emissiontype semiconductor laser made of GaAlAs materials, the other III-V groupcompounds may be equivalently used in the present invention.

Although this embodiment has been described in connection with thestructure shown in FIG. 6 and the light emitting portion shown in FIG.8(c), the present invention is not limited to such an arrangement.

FIGS. 9a to 11c show the other embodiments of the present invention inwhich various configurations and arrangements of optical resonators andassociated separation grooves in a plane parallel to the substrate asviewed from the exit side are schematically illustrated. FIGS. 9(a)-(j)and (m) represent line symmetry arrangements in which a plurality ofcolumn-like semiconductor layers each having a circular or regularlypolygonal cross-section parallel to the substrate are formed. In anyevent, the light emitting spot formed by any one of such arrangementscan have a dimension larger than that of a light emitting spot formed bya single light emitting portion. When it desired to provide a singlecircular cross-sectional laser beam having a relatively large diameterfrom the respective light emitting portions and separation groove, thecross-section of each of the light emitting portions may be anyconfiguration other than circle or regular polygon. The essentialrequirement in the concept of the present invention is that anon-circular or non-polygonal line joining the outer edges of the lightemitting portions arranged in line symmetry approximate to a circular orregularly polygonal configuration. So the configurations shown in FIGS.(k) and (l) are suitable for generating the laser beam having largerdiamiter. Each of embodiments shown in FIGS. 10(a)-(d) and 11(a)-(c)includes light emitting portions of n in number and is advantageous inthat it can produce a light emitting spot formed into any desirable sizeand form, in addition to the same advantages as in the embodiment ofFIG. 6. In all the embodiments shown in FIGS. 10a-11c, a line beam maybe provided by disposing a plurality of light emitting portions in rowand/or column on a two-dimensional plane parallel to the substrate.

In the embodiment shown in FIG. 6, there may be produced a semiconductorlaser which comprises a plurality of spaced p-type ohmic electrodes 310equal in number to the light emitting portions 320, these electrodes 310being connected with the contact layer 308. In such a case, each of thelight emitting portions will generate a beam having a circularcross-section which can be independently controlled in ON, OFF andmodulation, these beams being synchronized with one another in phase.

It is to be understood that the surface emission type semiconductorlaser of the present invention may be equivalently applied to anydesirable light source in various devices such as printer, copyingmachine, facsimile, display and so on.

We claim:
 1. A surface emission type semiconductor laser for emitting alaser beam in a direction perpendicular to a semiconductor substrate inwhich said laser is formed, said semiconductor laser comprising:opticalresonator means including a pair of reflecting mirrors having differentreflectivities and a plurality of semiconductor layers between saidreflecting mirros, the semiconductor layers including at least acladding layer in said semiconductor layers being formed into at leastone column-like portion; a II-VI group compound semiconductor epitaxiallayer surrounding said at least one column-like portion; and an activelayer of multi-quantum well structure formed at a lower layer section ofsaid cladding layer having the column-like structure, said active layerincluding a plurality of well layers each of which is sandwiched betweenbarrier layers.
 2. A surface emission type semiconductor laser asdefined in claim 1 wherein said II-VI group compound epitaxial layer isformed of a combination of two, three or four elements selected from IIgroup elements, Zn, Cd and Hg and from VI group elements, O, S, Se andTe.
 3. A surface emission type semiconductor laser as defined in claim 1wherein said II-VI group compound semiconductor epitaxial layer has alattice constant corresponding to a lattice constant of said at leastone column-like portion.
 4. A surface emission type semiconductor laseras defined in claim 1 wherein a cross-section of said at least onecolumn-like portion parallel to said semiconductor substrate is of acircular or regularly polygonal configuration.
 5. A surface emissiontype semiconductor laser as defined in claim 4 wherein said at least onecolumn-like portion has either of a diameter or diagonal lines equal toor less than 10 μm in planes parallel to said semiconductor substrate.6. A surface emission type semiconductor laser as defined in claim 1wherein a semiconductor contact layer of said optical resonator means onan exit side thereof has a thickness equal to or less than 3.0 μm.
 7. Asurface emission type semiconductor laser as defined in claim 1, furthercomprising a waveguide layer formed at a lower layer section of saidactive layer of multi-quantum well structure, said waveguide layeradapted to propagate light rays in a direction parallel to said activelayer.
 8. A surface emission type semiconductor laser as defined inclaim 7 wherein a refractive index of said waveguide layer is lower thanan equivalent refractive index of said active layer but higher than arefractive index of a cladding layer below said waveguide layer.
 9. Asurface emission type semiconductor laser as defined in claim 1 whereinsaid optical resonator means includes one column-like portion andwherein one of said reflecting mirrors on a exit side is formed oppositeto an end face of said column-like portion within the range of said endface.
 10. A surface emission type semiconductor laser as defined inclaim 9 wherein said optical resonator means has a rib waveguide typerefractive index waveguide structure, said rib waveguide being formed bysaid active layer of multi-quantum well structure.
 11. A surfaceemission type semiconductor laser as defined in claim 9 wherein saidcolumn-like portion includes said active layer of multi-quantum wellstructure to form a buried type refractive index waveguide structure.12. A surface emission type semiconductor laser as defined in claim 1wherein said optical resonator means includes separation groove meansfor separating a plurality of said column-like portions from oneanother, said II-VI group compound semiconductor epitaxial layer beinglocated in said separation groove means to form a light emitting portionon each of said column-like portions, and wherein said active layer isformed as a common layer for all the light emitting portions, wherebylight beams from said light emitting portions can be synchronized withone another in phase.
 13. A surface emission type semiconductor laser asdefined in claim 12, further comprising a waveguide layer formed at alower layer section of said active layer of multi-quantum wellstructure, said waveguide layer adapted to propagate light rays in adirection parallel to said active layer.
 14. A surface emission typesemiconductor laser as defined in claim 13 wherein a refractive index ofsaid waveguide layer is lower than an equivalent refractive index ofsaid active layer but higher than a refractive index of a cladding layerbelow said waveguide layer.
 15. A surface emission type semiconductorlaser as defined in claim 12 wherein said separation groove means hasside walls extending perpendicular to said semiconductor substrate. 16.A surface emission type semiconductor laser defined in claim 12 whereinsaid II-VI group compound semiconductor epitaxial layer is transparentfor the wavelength of the emitted laser beam and wherein the reflectingmirror on the exit side is formed over a region opposite to end faces ofsaid column-like portions and to said II-VI group compound semiconductorepitaxial layer located in said separation groove means.
 17. A surfaceemission type semiconductor laser as defined in claim 16 wherein saidII-VI group compound semiconductor epitaxial layer is formed of any oneselected from a group consisting of ZnSe, ZnS, ZnSSe, ZnCdS and CdSSe.18. A surface emission type semiconductor laser as defined in claim 16wherein a width of said separation groove means in a direction parallelto said semiconductor substrate is equal to or more than 0.5 μm and lessthan 10 μm.
 19. A surface emission type semiconductor laser as definedin claim 16, further comprising a waveguide layer formed at a lowerlayer section of said active layer of multi-quantum well structure, saidwaveguide layer adapted to propagate light rays in a direction parallelto said active layer.
 20. A surface emission type semiconductor laser asdefined in claim 19 wherein a refractive index of said waveguide layeris lower than an equivalent refractive index of said active layer buthigher than a refractive index of a cladding layer below said waveguidelayer.
 21. A surface emission type semiconductor laser as defined inclaim 16 wherein each of said column-like portions has a circular orregularly polygonal cross-section in a two-dimensional plane parallel tosaid semiconductor substrate and wherein a said plurality of saidcolumn-like portions are arranged in line symmetry in saidtwo-dimensional plane to emit a laser beam having a circularcross-section.
 22. A surface emission type semiconductor laser asdefined in claim 16 wherein each of said column-like portions has anon-circular or non-polygonal cross-section on a two-dimensional planeparallel to said semiconductor substrate and wherein outer edges of saidcolumn-like portions are arranged to form substantially a circular orregularly polygonal profile, whereby a laser beam having a circularcross-section can be emitted from the semiconductor laser.
 23. A surfaceemission type semiconductor laser as defined in claim 16 wherein aplurality of said optical resonator means each defined by a plurality ofsaid column-like portions are formed on said semiconductor substrate toprovide an independent electrode in each of said optical resonator meanson the exit side, whereby a laser beam emitted from each of said opticalresonator means and having a circular cross-section capable of formingan increased light emitting spot can be independently controlled in ON,OFF and modulation.
 24. A surface emission type semiconductor laser asdefined in claim 16 wherein a plurality of said column-like portions areequidistantly arranged in row and/or column to provide a laser beamemitted therefrom in the form of a line beam.